1. Field of the Invention
This invention relates to a method for the deposition of metals in bacterial cellulose and the utilization of the metallized bacterial cellulose in the construction of fuel cells and other electronic devices.
2. Description of the Related Art
The production of bacterial cellulose (also referred to as microbial cellulose) from cellulose-synthesizing bacterium has been studied for over half a century. For instance, it was reported back in 1947 that in the presence of glucose and oxygen, resting cells of Acetobacter xylinum synthesize cellulose (see, S. Hestrin et al., “Synthesis of Cellulose by Resting Cells of Acetobacter xylinum”, Nature 159: 64-65, 1947).
Through subsequent studies, it was determined that the physical properties of bacterial cellulose differ from those of the cellulose produced by green plants. Upon visual examination, it is evident that plant and bacterial cellulose differ in appearance and water content. Plant cellulose has a fibrous structure, while bacterial cellulose resembles a gel. In its hydrated state, the bacterial cellulose contains over a hundred times its weight in water. Yet both of these substances are built from the same basic unit, chains of glucose molecules that are linked by β-1,4-glycosidic bonds. The difference in the properties of these materials results from their nanoscale structural architecture. Cellulose that is synthesized by plants such as cotton (Gossypium spp.) and ramie (Boehmeria nivea) has a structure resembling a heavy-duty rope made of many small fibers twisted into larger fibers that are then twisted into the rope. Thirty-six glucose chains are assembled into an elementary fibril with a diameter of 3.5 nanometers. Microfibrils are assembled into macrofibrils that have a diameter ranging from 30 to 360 nanometers. The macrofibrils are then assembled into fibers. Imaging of cotton linter fibers by atomic force microscopy found an average macrofibril diameter of approximately 100 nanometers (see, Hon, “Cellulose: a random walk along its historical path”, Cellulose 1:1-25 1994; and Franz et al. “Cellulose”, in Methods in Plant Biochem. Vol. 2, Chapter 8, P. M. Dey and J. B. Harborne, editors, Academic Press, London, pages 291-322, 1990).
The most widely studied cellulose-synthesizing bacterium is Acetobacter xylinus (formerly Acetobacter xylinum, recently renamed Gluconacetobacter xylinus according to the American Type Culture Collection). In fact, this microorganism has been used for the production of the food product nata de coco in the Philippines. Cellulose is secreted by Acetobacter in the form of a twisted ribbon 40 to 60 nanometers wide that is extruded at a rate of 2 micrometers/minute. Each ribbon consists of 46 microfibrils, each of which has an average cross-section of 1.6×5.8 nanometers. These twisted ribbons, roughly corresponding to the macrofibrils of plant cellulose, assemble into sheets outside the cell, that combine to form a centimeter-thick layer called a pellicule on the surface of the culture medium. Scanning electron microscopy has revealed that, inside the pellicule, the fibrils are organized to form tunnels with a diameter of 7 micrometers, large enough for the bacteria to move through (see, S. Hestrin et al., “Synthesis of Cellulose by Resting Cells of Acetobacter xylinum”, Nature 159: 64-65, 1947; S. Hestrin, et al., “Synthesis of cellulose by Acetobacter xylinum: Preparation of freeze-dried cells capable of polymerizing glucose to cellulose”, Biochem. J. 58: 345-352, 1954; and Cannon et al., “Biogenesis of Bacterial Cellulose”, Crit. Reviews in Microbiol. 17(6): 435-447, 1991).
The aforementioned nata de coco or coconut gel has been produced for domestic consumption in the Philippines for at least 100 years. Nata de coco is the gel-like cellulose pellicule formed on the surface of media by Acetobacter xylinum cultures. In recent years, it has become one of the most popular Filipino food exports. The export of nata de coco grew from $1.0 million in 1992 to $25.9 million in 1993, with 95% of the total going to Japan. Traditional production of nata de coco is carried out in the Philippines as a cottage industry. Fermentation of coconut milk and glucose medium inoculated with starter is carried out under static culture conditions, i.e., in square plastic containers 1.5 centimeters high. The fermentation broth is acidified by the addition of acetic acid. Typically, a fermentation time of 10 to 12 days at ambient temperature is required for production of a layer or pellicule 1 centimeter thick. The pellicules are washed with water and, in some cases, sodium hydroxide solution, then cut into 1 centimeter cubes. The cubes are generally soaked in sucrose solutions with addition of flavorings and colors for the food product (see, Technology and Livelihood Resource Center, Makati City, Philippines, available on the Internet at http://esprint.com.ph/cocosoy/mainpage/mainpage.html).
The unique properties of the bacterial cellulose synthesized by Acetobacter have inspired attempts to use it in a number of commercial products. These include tires (see, e.g., U.S. Pat. No. 5,290,830), headphone membranes (see, e.g., U.S. Pat. No. 4,742,164), paper (see, e.g., U.S. Pat. No. 4,863,565), and textiles (see, e.g., U.S. Pat. No. 4,919,753). Medical applications include a specially prepared membrane to be used as a temporary skin substitute for patients with large burns or ulcers (see, e.g., U.S. Pat. No. 4,912,049, and Fontana, et al., “Acetobacter Cellulose Pellicule as a Temporary Skin Substitute”, Appl. Biochem. Biotech. 24/25: 253-264 12, 1990). A patent has also issued on the possible use of bacterial cellulose preparations as a source of dietary fiber (see, e.g., U.S. Pat. No. 4,960,763). Numerous patents have issued on the production of bacterial cellulose modified in some manner during cell growth or during processing (see, e.g., U.S. Pat. Nos. 5,079,162, 5,871,978, 6,060,289, 5,955,326, 5,962,277, 5,962,278, 6,017,740, and 6,071,727). It has also been reported that the addition of certain dyes to the culture medium inhibits the assembly of the pellicule sheets (see, Brown et al., “Experimental Induction of Altered Nonmicrofibrillar Cellulose”, Science 218: 1141-1142, 1982), and that the addition of carboxymethylcellulose to the medium results in the formation of cellulose with special optical properties (see, e.g., U.S. Pat. No. 4,942,128).
In addition to studies directed to bacterial cellulose and it uses, others have studied the chemical reactions of cellulose more generally. For instance, various mechanisms of cellulose hydrolysis has been studied and reported on by Lassig, Shultz, Gooch, Evans and Woodward in “Inhibition of Cellobiohydrolase I from Trichoderma reesei by Palladium”, Arch. Biochem. Biophys. 322:119-126, 1995, and in “Palladium—a new inhibitor of cellulase activity”, Biochem. Biophys. Res. Comm. 209: 1046-1052, 1995. As part of research studying the mechanism of cellulose hydrolysis, several metal ions and complexes were tested for inhibition of cellobiohydrolase I (CBH I), the β-1,4-glucan hydrolase comprising the major component of the cellulase mixture secreted by the fungus Trichoderma reesei. The most important contribution of CBH I to the hydrolysis of crystalline cellulose appeared to be the binding and disruption of the cellulose fibers. Specifically, a compound was sought that would inhibit hydrolysis of the β-1,4-glycosidic bond of the cellulose chains but would not effect binding to the crystalline cellulose. Sodium and ammonium hexachloropalladate were found to be the most effective inhibitors of the compounds screened. The hexachloropalladate inhibited hydrolysis of both small soluble substrates and crystalline, insoluble cellulose by CBH I, but did not inhibit binding of the enzyme to the insoluble cellulose.
For years, cellulose-containing products in general have also been studied for uses as an alternative source of fuel. For instance, the conversion of biomass to energy has been studied for some time. Increased use of cellulose-containing products in heat or electricity (power) generation systems would be particularly desirable as products which contain cellulose are a renewable resource. However, the use of bacterial cellulose in power generation systems has not been investigated.
One power generation system that has attracted widespread interest is the fuel cell. There are different types of fuel cells, but they each produce electrical energy by means of chemical reaction. One type of fuel cell is the polymer electrolyte membrane fuel cell which comprises a polymeric electrolyte membrane sandwiched between an anode and a cathode. The fuel cell generates electrical power by bringing a fuel into contact with the anode and an oxidant into contact with the cathode. The fuel is typically a hydrogen-containing material (for example, water, methane, methanol or pure hydrogen), and may be supplied to the fuel cell in liquid form or gaseous form, such as hydrogen gas. The fuel is introduced at the anode where the fuel reacts electrochemically in the presence of a catalyst to produce electrons and protons in the anode. The electrons are circulated from the anode to the cathode through an electrical circuit connecting the anode and the cathode. Protons pass through the electrolyte membrane (which is an electron insulator and keeps the fuel and the oxidant separate) to the cathode. Simultaneously, an oxygen-containing oxidant, such as oxygen gas or air, is introduced to the cathode where the oxidant reacts electrochemically in the presence of a catalyst consuming the electrons circulated through the electrical circuit and the protons at the cathode. The halfcell reactions at the anode and the cathode are, respectively: H2→2H++2e− and ½O2+2H++2e−→H2O. The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate halfcell reactions written above.
While fuel cells are highly efficient electrochemical energy conversion devices that directly convert the chemical energy derived from renewable fuels into electrical energy, they do have disadvantages. Specifically, long felt needs generally exist to reduce initial costs and provide for inexpensive maintenance of fuel cell installations. The high cost of fabricating the fuel cells is due to many factors including (among other things) the high cost of synthetic polymeric materials used in the electrodes and the electrolyte membranes, the safety and environmental measures necessary for safe manufacture of the electrodes and electrolyte membranes, the difficulties in controlling the concentration of expensive catalysts, and the problems associated with the bonding of electrodes and the electrolyte membrane. Furthermore, it can be quite expensive to replace worn out fuel cells and recover the anode and cathode catalysts that may become fouled during operation. Also, fuel cell materials may not be amenable to recycling because of the presence of metal catalysts.
Therefore, there is a need for a fuel cell power generation system that can be fabricated from components that are inexpensive to manufacture and that can be readily recycled and recovered. In particular, it would be beneficial if these fuel cell components could be manufactured from a renewable resource such as a natural cellulose containing material.